Gas generators are refined, mature products which react a hydrocarbon fuel (typically methane) with an oxidant (typically air) to produce a gas stream having a defined range of components. Generally speaking, generators can be classified as exothermic or endothermic depending on whether the reaction between hydrocarbon fuel and oxidant is viewed as liberating heat (exothermic) or consuming heat (endothermic). Typically, an endothermic generator passes the fuel and oxidant through a packed catalyst bed and reference can be had to Schultz U.S. Pat. No. 4,805,881 for a discussion of the reactions and heat balances through the bed. The same type of reactions occur in exothermic gas generators which do not, generally speaking, have a catalyst bed. Reference can be had to Knight U.S. Pat. No. 3,208,830 for a showing of an exothermic gas generator design still in commercial use today.
Generally, the gas composition of air is fixed and principally comprises oxygen and nitrogen which will produce when reacted with a hydrocarbon fuel, such as natural gas, a resultant gas mixture (products of combustion) having varying percentages of certain gases depending upon the ratio of air to fuel which is reacted. When the air to natural gas is fixed at a ratio of about 9.2 parts of air to 1 part of natural gas (by volume) stoichiometric combustion occurs which means the products of combustion contain little if any combustibles, i.e., H.sub.2 and CO. In the trade, gas generators are typically supplied with a variable turn-down ratio so that the gas generator can be operated "rich" or "lean". Turn-down ratio simply means the ratio of air to fuel. "Lean" or "excess air" means operating the generator at close to stoichiometric conditions and "rich" means operating the generator at less than stoichiometric ratios. It is of course appreciated, that the products of combustion in turn will undergo equilibrium at whatever temperature they are at (and while undergoing equilibrium will in turn experience certain kinetic reactions) which in turn will affect the percentages of certain gases which make up the products of combustion. In the trade and as used in this specification, products of combustion means the resultant gas initially produced when the hydrocarbon fuel is reacted or combusted with an oxidant at its reaction temperature. When the products of combustion achieve compositions and composition percentages at equilibrium at a desired temperature, the gas is referred to as a product gas.
Endothermic gas generators are typically operated very rich at low air to fuel ratios of about 2.5 to 1. The water gas shift reaction is controlled to equilibrium in the catalyst bed to produce a product gas, such as American Society of Metal Class 300 carrier gas which has trace amounts of water vapor. As a practical matter the invention has little application for endothermic gas generators when used to produce gases for effecting heat treating processes such as carburizing.
Exothermic gas generators, because of the exothermic reaction, are typically operated at higher air-fuel ratios (from a rich condition of anywhere between about 5-7 parts of air to 1 part of natural gas to stoichiometric) then endothermic gas generators and produce products of combustion having a relatively high, initial percentage of water vapor (about 12-15%), and is a "wet gas". The gases produced are classified as inert gases because they normally do not contain combustibles in that amount produced by endothermic gas generators. However, when exothermic generators are operated "rich", they can produce combustibles as high as 20%. This is desirable for annealing heat treat processes in which the furnace atmosphere is desired to have some hydrogen which is beneficial to the annealing process while possessing the ability to produce an inert gas for purging purposes (also required not only in the annealing process but in the operation of any furnace).
In the conventional exothermic gas generator the gas is immediately cooled from its combustion temperature in excess of about 2,800.degree. F. to a temperature of about 100.degree. F. by passing through a water spray within the generator such as shown in the Knight patent followed by further passing through a water tower or chiller. The composition of the product gas is optionally further changed by cooling or drying the gas further by subsequently passing the 100.degree. F. gas over refrigeration coils to lower the dew point (temperature at which condensation of water vapor in the product gas takes place) to about 40.degree. F. A typical product gas composition by volume percent produced by an exothermic gas generator marketed by the assignee Surface Combustion under the brand name DX.RTM. operated at lean and rich conditions is set forth in the published table below.
______________________________________ INERT RICH 100.degree. F. 40.degree. F. 100.degree. F. 40.degree. F. DX .RTM. D.P. D.P. D.P. D.P. ______________________________________ CO.sub.2 10.4 11.0 4.7 5.0 CO 0.5 0.5 9.4 10.0 H.sub.2 0.5 0.5 9.4 10.0 H.sub.2 O 6.5 0.8 6.5 0.8 N.sub.2 82.1 87.2 69.6 73.8 ______________________________________
In general summary, the products of combustion which are initially formed at temperatures in excess of about 2,800.degree. F. are cooled in the generator and water towers to about 100.degree. F. and then passed through refrigeration coils where the water vapor condenses on refrigeration coils to dew points of about 40.degree. F. There are special heat treat processes where the products of combustion are desired to be dried to dew points less than 40.degree. F. and preferably less than or about 20.degree. F..
Because of the phase change of water to ice at temperatures of 32.degree. F. it is not feasible to use conventional refrigeration cycles to achieve drying of the "wet" products of combustion to dew points lower than 32.degree. F. This is because ice from the water vapor condenses on the refrigerant coils limiting the temperature at which the product gas can be cooled to 32.degree. F. The refrigerant system then has to operate in a defrost mode to remove the ice.
Within the gas generator art, it is conventionally known to pass the gas either through alternately used chambers of a known desiccant material such as alumina or through alternately used molecular sieve beds. In either of these arrangements, one of the beds is "rejuvenated" or "reclaimed" by boiling off the saturated water and other impurities while the other bed is on-line drying the gas to temperatures where the dew point of the gas is less than 32.degree. F. When the on-line bed becomes contaminated from the products of combustion, the system switches to the reclaimed bed during which time the on-line bed (desiccant or molecular sieve) is reclaimed. An example of such a system is set forth in Beggs U.S. Pat. No. 2,712,981 incorporated by reference herein.
In practice such systems as molecular sieve and desiccant dryers have been found to have an inherent and immediate drop off in performance as they become saturated with water and other impurities requiring reclamation to boil off the water etc., at increasingly frequent cycles. Eventually the arrangement tends to approach a continuous type of cycle requiring replacement of the desiccant or molecular sieves. Significantly, as a function of time, the reclamation or boiling off phase simply does not completely refurbish the absorbent material in the dryer and the performance curve of the system shifts. This makes control of the system, ie., the dew point of the gas, increasingly difficult to attain and maintain.